Field experiments were conducted in 2017 and 2018 at two locations in Indiana to evaluate the influence of cover crop species, termination timing, and herbicide treatment on winter and summer annual weed suppression and corn yield. Cereal rye and canola cover crops were terminated early or late (2 wk before or after corn planting) with a glyphosate- or glufosinate-based herbicide program. Canola and cereal rye reduced total weed biomass collected at termination by up to 74% and 91%, in comparison to fallow, respectively. Canola reduced horseweed density by up to 56% at termination and 57% at POST application compared to fallow. Cereal rye reduced horseweed density by up to 59% at termination and 87% at POST application compared to fallow. Canola did not reduce giant ragweed density at termination in comparison to fallow. Cereal rye reduced giant ragweed density by up to 66% at termination and 62% at POST application. Termination timing had little to no effect on weed biomass and density reduction in comparison to the effect of cover crop species. Cereal rye reduced corn grain yield at both locations in comparison to fallow, especially for the late-termination timing. Corn grain yield reduction up to 49% (4,770 kg ha–1) was recorded for cereal rye terminated late in comparison to fallow terminated late. Canola did not reduce corn grain yield in comparison to fallow within termination timing; however, late-terminated canola reduced corn grain yield by up to 21% (2,980 kg ha–1) in comparison to early-terminated fallow. Cereal rye can suppress giant ragweed emergence, whereas canola is not as effective at suppressing large-seeded broadleaves such as giant ragweed. These results also indicate that early-terminated cover crops can often result in higher corn grain yields than late-terminated cover crops in an integrated weed management program.

Glyphosate-resistant giant ragweed has become an increasing problem, and the potential spread of these biotypes is a threat to production agriculture and to the long-term utility of glyphosate and glyphosate-resistant crops. The fate of glyphosate resistance in a giant ragweed population is dependent on the fitness of the resistant biotype. Our objective was to determine the fitness of glyphosate-resistant giant ragweed in the absence and presence of glyphosate. We compared the growth and seed production of glyphosate-resistant (GR) and glyphosate-susceptible (GS) giant ragweed under field conditions in the absence of glyphosate. A greenhouse study was also conducted to determine the survival of GR and GS giant ragweed and their open-pollinated progeny from the field study under glyphosate-induced selection pressure. In the absence of glyphosate, GR giant ragweed displayed rapid, early season growth, but 50 d after planting, the biotypes were similar in height, shoot weight, and leaf area. During reproduction, the GR biotype flowered earlier and produced 25% less seed than the GS biotype. In the presence of glyphosate, an outcrossing rate of 31% was detected between GR and GS biotypes because 61% of progeny were resistant to glyphosate at 840 g ae ha−1. A second application 14 d later at 2,520 g ae ha−1 completely removed the GS alleles from the population, whereas several homozygous and heterozygous GR plants survived and produced seed. These results indicate GR will persist in the population when subjected to glyphosate and that glyphosate resistance in giant ragweed has the potential to spread rapidly in our current agricultural ecosystem.

Robust predictions of weed seedling emergence from the soil seedbank are needed to aid weed management. A common seed accession (Illinois) of giant ragweed was buried in replicate experimental gardens over 18 site years in Illinois, Michigan, Kansas, Nebraska, Ohio, and South Dakota to examine the importance of site and climate variability by year on seedling emergence. In a nonlinear mixed-effects modeling approach, we used a flexible sigmoidal function (Weibull) to model giant ragweed cumulative seedling emergence in relation to hydrothermal time accumulated in each site-year. An iterative search method across a range of base temperature (Tb ) and base and ceiling soil matric potentials (ψb and ψc) for accumulation of hydrothermal time identified optima (Tb = 4.4 C, ψb = −2,500 kPa, ψc = 0 kPa) that resulted in a parsimonious regional model. Deviations between the fits for individual site-years and the fixed effects regional model were characterized by a negative relationship between random effects for the shape parameter lrc (natural log of the rate constant, indicating the speed at which emergence progressed) and thermal time (base 10 C) during the seed burial period October through March (r = −0.51, P = 0.03). One possible implication of this result is that cold winter temperatures are required to break dormancy in giant ragweed seeds. By taking advantage of advances in statistical computing approaches, development of robust regional models now is possible for explaining arable weed seedling emergence progress across wide regions.